Many electric machines, such as electric motors and electric generators, include a stator that is held stationary and a rotor that rotates adjacent the stator. The stator and rotor may be configured to transfer power between one another through one or more magnetic fields. Some electric machines may include an interior permanent-magnet-type rotor with permanent magnets mounted inside a rotor core of the rotor for generating a magnetic field of the rotor. Such a rotor may have the permanent magnets arranged in a manner such that they form north and south magnetic poles of the rotor in alternating positions around the circumference of the rotor. For example, at each of multiple equal angular intervals, the rotor may include a single magnet that forms either a north or south magnetic pole of the rotor. Such a rotor may have no magnets disposed between the ends of the magnets forming adjacent north and south magnetic poles.
Thus, the intensity of the magnetic field generated by the rotor may vary as a function of the position around the circumference of the rotor. At those circumferential positions occupied by a magnet forming a north or south magnetic pole of the rotor, the magnetic field may have an intensity substantially equal to that generated by the magnet. At the end of each magnet providing a magnetic pole of the rotor, the intensity of the magnetic field of the rotor may drop in a substantially stepped manner, with the magnetic field having very low intensity at circumferential positions between the ends of adjacent magnets. The manner in which the intensity of the magnetic field of the rotor varies as a function of circumferential position may impact various performance characteristics of the electric machine, including the magnitude of eddy current losses and torque ripple experienced by the machine. Unfortunately, with a single step change in magnetic field intensity at the end of each magnetic pole, a permanent-magnet-type rotor that employs a single permanent magnet for each magnetic pole may significantly limit the ability to tailor the distribution of magnetic flux intensity around the circumference of the rotor.
Published U.S. Patent Application No. 2007/0145850 A1 to Hsu (“the '850 application”) discloses an electric machine having a rotor with multiple permanent magnets at each magnetic pole of the rotor. The rotor disclosed by the '850 application includes a rotor body formed by a plurality of laminations of ferromagnetic material clamped together on a hub. At each magnetic pole of the rotor, pole pieces secure a plurality of permanent magnets to this rotor body.
Although the '850 application discloses an electric machine with a rotor having multiple permanent magnets at each of its magnetic poles, certain disadvantages persist. For example, the '850 application includes no disclosure regarding how to tailor the relative positions of the permanent magnets to reduce eddy current losses or torque ripple.
The electric machine of the present disclosure solves one or more of the problems set forth above.